1. Opisz budowę lasera światłowodowego.
Fiber Laser Components
Several components are critical in making a good fiber laser. Each component has specific
challenges:
•
Pump Diodes: A sufficiently bright and powerful fiber-coupled pump source is needed. This
requires producing high-brightness diodes with high reliability, at a very low cost and in
large volumes. Achieving good yield in production is crucial.
•
Active Fiber: To provide gain, a robust doped fiber that can handle the power (that is, does
not suffer from photodarkening) is required. The active fiber must have very low losses and
no imperfections that might cause scattering and reduce the beam quality. For pulsed
applications, it is important to reach high doping levels to reduce active fiber length and
prevent nonlinearities. The fiber also must have very good splicing properties, ensuring low
loss connections to other fiber components.
•
Mirrors: Fiber Bragg gratings (FBGs) that make the mirrors for the cavity must handle high
power levels without optical degradation (in many cases, the pump power goes through the
FBG in addition to the signal) and must have good splicing properties.
•
Pump Combiner: The pump light is typically brought into the active fiber using power
combiners, making it possible to use a large number of fiber-coupled diodes for pumping.
These combiners must be able to handle high levels of pump power, and must exhibit very
low loss (losses cause heat, so any excess loss spells trouble). Most pump combiners are
made with a complicated and fairly nonscalable manufacturing process, so being able to
make these components at a low cost and high volume is a tough challenge.
•
Isolators: Good isolators are crucial for protection against back-reflections. This is
important for pulsed fiber lasers, because of the extreme peak powers they produce.
Continuous-wave (CW) or modulated CW typically does not require isolators. Isolators also
may be needed to protect the pump diodes from feedback caused by intracavity pulses.
Given that most pulsed fiber lasers use master oscillator-power amplifier (MOPA)
architecture, isolators also often serve an important role in separating the gain stages from
each other.
•
Output Combiner: For high-power CW, yet another type of combiner is needed, currently
not readily available in the open market. This combiner superimposes the power of several
(typically about 400 to 800 W) fiber laser submodules to create kilowatt-class output. Here,
extremely low losses are absolutely crucial, as well as minimal degradation of the final
output beam quality.
•
Power Supply: High-power CW lasers are increasingly applied in modulated mode, which
means they are rapidly switched on and off by modulating the pump diode current. While
both diodes and the fiber can handle this modulation without difficulty, modulation puts
significant stress on the electronic current sources used to drive the diodes. Highly reliable
current sources for diode modulation are crucial.
•
Beam Delivery: Finally, to get the fiber laser output to the workpiece, passive transfer fiber
(and cable) is needed, in addition to robust fiber connectors and beam switches. Low loss
and beam quality preservation (or adaptation) are key.
2. Wymień możliwe rozwiązania konstrukcji rezonatorów w laserze
światłowodowym.
Fiber Laser Resonators
In order to form a
laser resonator
with fibers, one either needs some kind of reflector (mirror) to
form a linear resonator, or one builds a fiber
ring laser
. Various types of mirrors are used in linear
fiber laser resonators:
•
In simple laboratory setups, ordinary
dielectric
mirrors
can be butted to the perpendicularly cleaved
fiber ends, as shown in Figure 1. This approach,
however, is not very practical for mass fabrication
and not very durable either.
•
The Fresnel reflection from a bare fiber end face is
often sufficient for the
output coupler
of a fiber laser.
Figure 2 shows an example.
•
It is also possible to deposit dielectric coatings
directly on fiber ends, usually with some evaporation
method. Such coatings can be used to realize
reflectivities in a wide range.
•
For commercial products, it is common to use
fiber Bragg gratings
, made either directly in
the doped fiber, or in an undoped fiber which is spliced to the active fiber. Figure 3 shows a
distributed Bragg reflector laser
(DBR laser) with two fiber gratings, but there are also
distributed feedback lasers
with a single grating in doped fiber, with a phase shift in the
middle.
Figure 3: Short DBR fiber laser for narrow-linewidth emission.
•
A better power-handling capability is achieved by collimating the
light exiting the fiber with a lens and reflecting it back with a
dielectric mirror (Figure 4). The
intensities
on the mirror are then
greatly reduced due to the much larger beam area. However, slight
misalignment can cause substantial reflection losses, and the
additional Fresnel reflection at the fiber end can introduce filter
effects and the like. The latter effects can be suppressed by using angle-cleaved fiber ends,
which however introduce polarization-dependent losses.
•
Another option is to form a fiber loop mirror (Figure 5), based on a
fiber coupler
(e.g. with 50:50 splitting ratio) and some piece of
passive fiber.
Most fiber lasers are pumped with one or several
fiber-coupled diode lasers
.
The pump light may be coupled directly into the core, or in
high-power
into
a larger pump cladding (→
double-clad fibers
), as discussed below in more detail.
There are many different kinds of fiber lasers, some of which are discussed in the following.
Figure 2: A simple erbium-doped
femtosecond laser, where the Fresnel
reflection from a fiber end is used for
output coupling.
Figure 4: End
reflector with lens
and mirror.
Figure 5: Fiber loop
mirror.
3. Wymień zalety laserów światłowodowych i ich ograniczenia
zalety:
trwałość (okres działania ok. 100 000 h)
solidna konstrukcja
proste chłodzenie
duża wydajność i sprawność
stabilność jakości wiązki laserowej
możliwość strojenia w dziedzinie częstotliwości
wiele długości fal (Tab. 1)
skalowalność (uzyskiwane moce do 10kW)
Ograniczenia:
*ograniczenie maksymalnej mocy
*są nowością na rynku, wobec czego:
-stanowią szczególne wyzwanie dla technologii
materiałowej
-wymagają znalezienia właściwych parametrów procesu
wytwarzania
-niezbędne jest zapoznawanie się środowisk społecznych z
nową technologią
*ograniczone wzmocnienie na jednostkę długości
*występują efekty nieliniowe
*justowanie źródeł wiązek pompujących falowód
na moje oko da pytanie nr 2.